A wealth of water worlds?

Storms cover the surface of a water world in this artist's conception. In the distance, a "hot Earth" and "hot Jupiter" appear close to the setting Sun. New computer simulations suggest such a scenario is plausible.

Nahks Tr 'Ehnl

September 11, 2006A series of state-of-the-art computer simulations show that earthlike planets form readily in the wake of a giant planet spiraling in toward its star. The authors of a new study estimate that more than a third of the nearly 200 known stars with giant planets may also harbor watery, Earth-size planets.

Forty percent of extrasolar planetary systems discovered so far sport "hot Jupiters" — giant planets that formed far from their star, but which, unlike Jupiter, fell inward for 100,000 years or so before settling into star-hugging orbits smaller than Mercury's.

Until now, most astronomers assumed a giant planet elbowing its way through a star's planet-forming dust disk would scatter the material needed to grow earthlike planets, halting further planet formation.

Sean Raymond, at the University of Colorado, Boulder, plus Steinn Sigurdsson and Avi Mandell, both at Pennsylvania State University, devoted 8 months of computer time to simulating the impact of a migrating gas giant on subsequent planet formation. Their results appear in the September 8, 2006, issue of Science.

They started with a gas and dust disk twice as massive as the one that spawned our solar system, but well within the known range of planet-forming disks. They seeded it with hundreds of planetesimals and Moon- to Mars-size planetary embryos. They then sent a Jupiter-size planet from a distant orbit down to one just a quarter the size of Earth's, and watched what happened for the next 200 million years.

Although the giant planet did fling many objects into highly eccentric orbits, gravitational resonances and drag from the protoplanetary disk eventually let planets form. In two of the four simulations, one Earth-mass planet formed inside the giant planet's orbit — a "hot Earth" — while a second formed outside, nestled in the star's habitable zone. "We have these big gas giants very close to the star," says Raymond, "but despite that, we have an earthlike planet around one astronomical unit [the average Earth-Sun distance], where life might develop."

A second surprise was the huge amount of water that rained onto the planets. The water took the form of icy, comet-like chunks from the disk's outer reaches. It was enough water to cover the planets to a depth of several miles. "I had no idea there would be so much water," says Raymond. "It opens a new realm of possibilities of where to look for life."

Other researchers cautiously support the findings. Richard Nelson, at Queen Mary University of London, says his group's simulations, still in press, show similar results. Still, he and several other experts emphasize that many different simulations will need to be run before we can know just how frequently stars harbor hot Jupiters and warm, wet Earths.

Robert Adler, based in Santa Rosa, California, is the author of Science Firsts: From the Creation of Science to the Science of Creation.